1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured by an inverse-stagger type or bottom gate type thin film transistor (hereinafter referred to as “TFT”) using a semiconductor film, and a method of manufacturing the semiconductor device. In particular, the present invention relates to a technique which is preferably applicable to an electro-optical device represented by a liquid crystal display device and an electronic device on which the electro-optical device is mounted. In the present invention, the semiconductor device is directed to all of devices that function using the semiconductor characteristics, and the electro-optical device and the electronic device on which the electro-optical device is mounted fall under the category of the semiconductor device.
2. Description of the Related Art
At present, in a note type personal computer (note personal computer) and a portable information terminal, a liquid crystal display device is employed for displaying an image or character information. Since an active matrix liquid crystal display device can obtain a high-fine image as compared with a passive liquid crystal display device, the active matrix liquid crystal display device is preferably employed for the above purpose. The active matrix liquid crystal display device is structured in such a manner that TFTs which function as active elements are arranged in a matrix in correspondence with the respective pixels in a pixel section. Each of those TFTs are normally formed of an n-channel TFT and controls a voltage which is applied to liquid crystal for each of the pixels as a switching element to conduct a desired image display.
There is the inverse-stagger type (or bottom gate type) TFT in which the active layer is formed of an amorphous semiconductor film. The amorphous semiconductor material is preferably formed of an amorphous silicon film. Since the amorphous silicon film can be formed on a large-area substrate at a low temperature of 300° C. or less, it is considered to be a material suitable for mass production. However, the TFT the active layer of which is formed of the amorphous silicon film is small in the field effect mobility to the degree of about 1 cm2/Vsec. Under the above circumstances, the drive circuit for conducting the image display is formed in an LSI chip and mounted by a TAB (tape automated bonding) system or a COG (chip on glass) system.
The active matrix liquid crystal display device thus structured is widely applicable to not only a note personal computer but also a 20-inch grade TV system, and demands for high precision and high aperture ratio have been increasingly raised in order to improve the image quality while a screen size has been large in area. For example, a document of “The Development of Super-High Aperture Ratio with Low Electrically Resistive Material for High-Resolution TFT-LCDs”, S. Nakabu, et al., 1999 SID International Symposium Digest of Technical Papers, pp. 732-735 has reported a technique of manufacturing a liquid crystal display device which is UXGA (1600×1200) in pixel density and 20 inches in size.
In order to supply and spread the above-mentioned products on market, there are required to improve the productivity and lower the costs while enhancing the reliability. In the active matrix liquid crystal display device, the TFTs are formed on a substrate by using a plurality of photomasks through the photolithography technique. In order to improve the productivity and also improve the yield, a reduction in the number of processes is considered as effective means. Specifically, it is necessary to reduce the number of photomasks required for manufacturing the TFTs. The photomask is used to form a photo-resist pattern on the substrate with a mask during an etching process in the technique of the photolithography. Therefore, if one photomask is used, there added processes for resist coating, pre-baking, exposing, developing, post-baking, etc., processes for forming a film, etching, etc., which are conducted before and after the former processes and processes for separating the resist, cleaning and drying, etc., resulting in a complicated work.